This invention relates to a varactor device of the type which can be used in an electronic tuner and, more particularly, to a semiconductor varactor device wherein plural varactor elements are formed in a common semiconductor body for improved shielding and minimal stray capacitance therebetween, the semiconductor varactor device being used as a component in a tuner, such as a UHF tuner.
In a conventional frequency tuner, such as the type used to receive broadcast frequencies in, for example, the UHF, VHF or FM band, it is necessary to vary the resonant frequency of the tuner in order to select a desired broadcast frequency. Typically, variable capacitor devices are used to adjust the frequency to which the tuner is tuned. Such variable capacitor devices have been the so-called air capacitor and, more recently, the varactor. A varactor is a voltage-controlled capacitive element whose capacitance is dependent upon a control voltage applied thereto. Thus, when the varactor is used as the resonant tuning element in a tuner, the frequency to which that tuner is tuned can be changed, or selected, by applying an appropriate control voltage to the varactor.
One problem in using varactors in high frequency tuners, such as a UHF tuner, is shielding the varactor from stray voltages which may deleteriously influence the effective capacitance thereof. Another problem has been minimizing stray capacitance between plural varactors when such plural varactors are used to tune various signal processing stages in the tuner. For example, a typical tuner may include an RF input stage, an RF amplifying stage, a local oscillating stage and a mixing stage. Some or all of these stages may be provided with an individual varactor element to be properly tuned to a selected frequency. Stray capacitances associated with these varactor elements tend to result in improper tuning of the tuner. This makes it difficult for a tuner to be used with automatic tuning apparatus without the intervention of a human operator.
Yet another problem attending the use of varactor devices in frequency tuners has been the requirement heretofore of highly skilled technicans to assemble such tuners. That is, in using the varactor elements to control the tuning frequency of the tuner, a common control voltage generally is applied to all of the varactor elements. This means that the voltage-capacitance characteristic of the varactor elements should be equal to each other. It has been found that a 2 to 3% deviation in the capacitance characteristic throughout the control voltage range is tolerable. However, since varactor devices which have been used heretofore may exhibit substantial differences in their voltage-capacitance characteristics, even if such varactor elements are manufactured from the same conductor wafer, it has been necessary to test individual varactor elements to obtain a matched set for use in a particular tuner. This laborious task of testing, as well as the assembly process, has resulted in generally high manufacturing costs for such tuners.